XML lets everyone create their own tags -- hidden labels such as or that annotate Web pages or sections of text on a page. It also allows users to add arbitrary structure to their documents but says nothing about what the structures mean.\\
Meaning is expressed by RDF, which encodes it in sets of triples, each triple being rather like the subject, verb and object of an elementary sentence. These triples \cite{1013501} can be written using XML tags \cite{1136011}. In RDF, a document makes assertions that particular things like people, Web pages etc have properties such as 'is a sister of,' 'is the author of' having certain values in the form of another person or  another Web page. Mostly, the data processed by machines follow the same structure. Subject and object are each identified by a Universal Resource Identifier (URI), just as used in a link on a Web page. The verbs are also identified by URIs, which enables anyone to define a new concept, a new verb, just by defining a URI for it somewhere on the Web.\\

But there could be a situation where two different databases represent the same concept using two different identifiers, such as ISBN. So, there should be some mechanism in which a program should be able to identify that these two terms are being used to mean the same thing. To handle this scenario, Ontologies comes into picture and it formally defines the relations among terms, classes and properties.  Hence, Semantic web helps in data integration across diverse systems easily and then allow us to query the system on the whole.
